Approaches such as MSe have proven useful in unbiased high sensitivity MSMS applications. In MSe the ion population is repeatedly switched between two or more modes of operation. In one of these modes of operation the ion population remains substantially un-fragmented and thus predominately consists of precursor ions. In another one of these modes, the ion population undergoes fragmentation and thus the resulting population predominately consists of product ions. The two or more populations are acquired using a mass spectrometer and product ions are assigned to precursor ions based on chromatographic precursor peak profile characteristics such as liquid chromatography retention time or peak shape. Whilst this approach has proven useful it does suffer from limited precursor ion to product ion assignment specificity in complex mixtures due to the relatively low peak capacity or resolution of the liquid chromatographic separation.
Data directed analysis (DDA) has conventionally been seen as an alternative approach to MSe. In DDA techniques, a MS survey scan is used to identify components of interest and once these components have been identified they are isolated using a mass filter, for example a quadrupole mass filter, before undergoing fragmentation and fragment ion mass analysis. The information in the fragment ion mass spectra, together with the precursor ion information, leads to a highly specific/selective experiment. However, the survey scan in a DDA technique is relatively slow and has a low duty cycle as only a single precursor ion can be analysed at a time.
It is therefore desirable to provide an improved method of mass spectrometry and an improved mass spectrometer.